It is paradoxical that, historically, that is to say before the 1950's, the largest elements in an electronic circuit tended to be the active elements. At that time, active elements took the form of expensive vacuum tubes. Vacuum tubes consumed copious amounts of power. Vacuum tubes operated at dangerously high voltages. Vacuum tubes generated very large amounts of heat. The cost of vacuum tubes dominated the parts cost of a product. Resistors and capacitors in those systems were relatively small. The effect of resistors and capacitors upon the size and cost of a product was almost negligible.
Today the situation is nearly exactly reversed. Semiconductor chips (integrated circuits) exist that include upward of 100 million transistors are commonplace. Transistors are the active elements in today's electronic devices that replace the vacuum tubes of pre-1950's electronics. Transistors are extremely small. Transistors consume microscopic amounts of power. Transistors operate at very low voltages. Transistors cost almost nothing. The effect of a single transistor on the design of a very large scale integration (VLSI) chip is virtually invisible. The paradox, then, is that resistors and capacitors now are relatively large when compared with the size of a single transistor.
Resistors and capacitors are very difficult to fabricate as part of integrated circuits. Even though resistors and capacitors may appear to be small to the naked eye, they occupy relatively large amounts of valuable area on an integrated circuit. This area might better be populated with transistors. For this reason, resistors and capacitors often appear in electronic circuits as discrete elements. Discrete elements are elements that stand alone on printed circuit boards outside VLSI chips.
Even so, situations do arise where it is necessary and desirable to fabricate, for example, a capacitor as part of a semiconductor integrated circuit. In these instances, every effort is made to keep the capacitor small and to squeeze it into as small an area as possible. The traditional textbook diagram of a basic capacitor shows two metal parallel plates (a first plate and a second plate). The plates are placed closely together with insulation (called dielectric material) between them. A terminal is attached to each plate. The capacitor functions by storing equal and opposite amounts of electrical charge on its opposite plates. A capacitor is characterized in part by its “capacitance.” Capacitance is a parameter that increases as the area of the plates increases and that further increases as the distance between the plates decreases. The value of the capacitance also is affected by the characteristics of the dielectric material that separates the two plates. Practical stand-alone capacitors can be fabricated, for example, from two sheets (a first sheet and a second sheet) of foil. The two sheets of foil are wrapped tightly together. An insulating layer is placed between the sheets. A terminal is attached to each sheet. The larger the area of the sheets and the thinner the insulating layer, the larger the capacitance. The picture to be grasped here is the concept that two metallic surfaces separated by insulating (dielectric) material form a capacitor.
Sometimes capacitance in a circuit is designed in on purpose and is therefore desirable. In other situations, capacitance arises incidentally. For example, any time two metallic surfaces lie close to each other, some amount of capacitance results. In integrated circuits, nearly invisible wires may run near each other for some distance. Even these tiny pairs of wires sometimes give rise to “parasitic” capacitance. Parasitic capacitance normally is considered to be undesirable. In still other situations, this effect of parasitic capacitance can be used to advantage. For example, a circuit may require a capacitor. If so, then one method for creating a capacitor comprises intentionally introducing some parasitic capacitance. Indeed, techniques for fabricating such capacitors have evolved to become quite sophisticated.
One form of capacitor fabricated in an integrated circuit by one technique is called a lateral metal-insulator-metal (LMIM) capacitor. An LMIM capacitor can be fabricated by constructing metallic fingers (like the tines of a fork) that interleave with each other to create a capacitive effect. The tines of one fork play the role of the first plate in the textbook version of a capacitor; the tines of the other fork play the role of the second plate. Some integrated circuits repeat this basic structure on several layers of the integrated circuit in order to increase the effective area of the plates of the capacitor being fabricated. Even so, only two groups of fingers are used, one group being connected together by wires or other means to form a first plate of the capacitor; the second group being connected together to form a second plate. When fabricated, the fingers of each plate resemble the bristles of a coarse hairbrush. The capacitor is formed by facing two “hairbrushes” together so that the two sets of “bristles” overlap while taking care to assure that the bristles of one hairbrush are insulated from the bristles of the other.
Although the mathematics can become complicated, design engineers are able to calculate the capacitance of these LMIM structures. By controlling the size, number of fingers, type of dielectric material and so on, a capacitor with a desired amount of capacitance can be designed quite precisely. However, while certain parasitic effects are exploited to design an LMIM capacitor, other parasitic effects act to frustrate the efforts of the circuit designer. These parasitic effects make it difficult to predict the value of capacitance that results in a given circuit design.
These other parasitic effects arise because of the proximity of an LMIM capacitor to other elements in the circuit that are not part of the LMIM capacitor itself. Thus, the capacitance of an LMIM capacitor can be influenced by factors outside the control of the LMIM capacitor designer.
The design of an integrated circuit is governed, in part, by a set of design rules to which circuit designers must adhere. One such set of rules governs the amount of metal that can appear in each layer of an integrated circuit. Integrated circuits typically are fabricated in layers with four, five, or six layers being employed in some embodiments. If a basic circuit design does not comprise sufficient metal to satisfy the design rules, then a “dummy fill” algorithm is used to add “islands” of metal in unused areas of each layer of the circuit in order to satisfy the design rules. Conversely, if a layer (such as a ground or power plane) comprises too much metal, then a “slotting” algorithm is employed to remove metal by creating slots in the existing metal, again, in order to satisfy the design rules. The slotting and dummy fill algorithms in the prior art do not take into account the structure of circuit elements near the slotting or dummy fill areas. In particular, the slotting and dummy fill algorithms insert and remove metal essentially randomly in the neighborhood of an LMIM capacitor. As a result, two identically designed LMIM capacitors placed in different regions of an integrated circuit can exhibit different performance because of the different parasitic effects present in each region. Further, the prior art gives the LMIM capacitor designer no means by which to correct this undesirable situation.